In the search for hydrocarbon bearing subsurface earth formations, various systems and methods have been devised for providing both qualitative information regarding the presence of such formations as well as quantitative information. It has been conventional to provide for a more qualitative analysis of various characteristics of hydrocarbon-bearing formation zones by various techniques, and then to follow up subsequently with other techniques conventionally thought to be more appropriate for quantitative analysis.
For example, it has long been known that valuable information can be obtained from actual sidewall core samples of formations at borehole depths of interest which may thence be analyzed for the presence of hydrocarbons. Although such techniques have frequently been relied upon in the past primarily for their qualitative nature, several problems have nevertheless diminished their value. First, core analysts frequently see such core samples long after gas has expanded out of the sample at the surface and been dissipated in the mud and after other fluids of the sample have been otherwise lost. Other samples may have been flushed in the formation. Such techniques suffer from yet a further debility in that core recoveries may average only 30-80% of footage cored, with most of the porous rock often being lost.
Yet another technique which has often been relied upon in like manner to coring primarily for the qualitative nature of the information provided is an operation well known in the art known variously as logging while drilling or measurement while drilling. In like manner to coring, the technique also sought to directly measure and detect presence of hydrocarbons by analysis of return drilling fluids during the drilling operation. Such techniques operated on the principle that as the drilling fluid circulated, it carried along cuttings suspended therein and derived from the formation through which it circulated. More particularly, it was fundamentally assumed that the hydrocarbons in the drilling fluids evolved from cuttings suspended therein which were picked up by the fluid while adjacent the drill bit. By measuring volume of hydrocarbons per sample volume of mud returned at the surface and correlating this to formation depth when the sample was adjacent the bit, a qualitative indication of the presence of hydrocarbons as a function of borehole depth was thus obtained.
As aforesaid, once the drilling operation was halted and core samples and/or logging while drilling or "mud logging" operations completed, it was conventional to thence run a log or suite of open hole well logs or "wireline" operations such as electric logs or the like well known in the art. The purpose of such measurements was to obtain more qualitative indications of parameters from which more detailed information about the hydrocarbon bearing potential of formations at various borehole elevations could be calculated. Such logging operations basically comprised the suspension of a logging instrument from a cable which was thence lowered into the borehole while various measurements were made as a function of depth. Such measurements might include the borehole diameter by means of a caliper, electrical potential between formation beds, radioactive impurities in the formation, electrical resistivity and conductivity of the wellbore, and the like. From these measured parameters, the desired formation information could be inferred.
Unfortunately, numerous problems associated with these logging techniques have continued to plague the industry. This is so notwithstanding the seeming inherent reliability due to the measurements being made in situ at the borehole elevations of interest rather than at the remote location of the well site surface (as in the case of the previously described coring and mud logging). However, such problems arise from the fact that the presence of hydrocarbons must be inferred from these wireline parameters unlike the case of mud logs and core samples wherein hydrocarbons are detected directly from the samples. Wireline measurements rely on measurements made out into the formation through the drilling fluid-filled borehole. Accordingly, such measurements may be inherently adversely affected to varying degrees by the mud filtrate itself in terms of thickness, salinity, etc., as well as borehole irregularities, inadequate pad contact, cycle skips of acoustic logs at fractures and the like. When borehole diameters vary, the mud cake thickness may vary contributing to the unreliability of these measured parameters. Numerous attempts have been made to compensate for these borehole effects as, for example, by measuring the borehole diameter (and presumably the mud thickness) with the caliper carried on the logging string. However, several difficulties are frequently encountered nevertheless. For example, in the case of borehole wall irregularities, formation fractures, and the like, the caliper may not be providing accurate measurements of mud filtrate thickness and borehole diameter as the filtrate may have invaded the formation to a substantial degree or the caliper may have suffered from radical excursions through the fractures. Moreover, such invasion may flush hydrocarbons away from the borehole, rendering the more remote detection thereof even more difficult.
Yet an additional serious problem with wireline measurements relates to associated borehole temperature and pressure effects. A surface volume of one cubic foot of hydrocarbon gas for example, when compressed at subsurface borehole elevations resulting in 11,000 psi borehole pressure may be compressed to as little as 1/550 of its original volume, or 0.0018 cubic feet, rendering it difficult to detect such minute gas particles in subsurface environments.
With the aforementioned limitations of subsurface logging in mind and returning now to a further discussion of the logging while drilling technique, one such operation may be seen disclosed in U.S. Pat. No. 2,328,555 entitled "Well Logging Method" to Herbert Hoover, Jr. This system illustrates some inherent deficiencies which have been overcome by the present invention. It will be recalled that in prior logging while drilling operations, they were employed often primarily for their qualitative indicators of potential hydrocarbon bearing formations or "shows" which were later quantitatively confirmed by other techniques such as wireline. Accordingly, such systems typically only calculated percentages of hydrocarbon gas per volume of return drilling fluid with no attempts to employ such measurements for quantitative formation reserves evaluation on a regular and continuous basis while drilling as, for example, in a volumetric analysis of hydrocarbon reserves per vertical foot drilled. Such prior art systems were content, as in the hereinbefore noted Hoover system, with a "batch" process analysis. In this system, a sample of drilling fluid was simply periodically withdrawn from the mud system and analyzed by a conventional gas chromatograph technique for relative presence of light and heavy hydrocarbons, or the like. The data was thence related to depth when the mud sample was adjacent the drill bit. Operators would, in fact, from time to time, calculate reserves from logging while drilling data for example by estimating average porosity values over formation intervals of interest wherein gross bed thicknesses of sand, for example, were simply "eyeballed". However, the prior art overlooked the possibility of obtaining direct downhole formation hydrocarbon reserve volumes on a regular basis per incremental borehole depth.
These and other deficiencies of the prior art have been overcome by the systems and methods of the present invention which will be described hereinafter in greater detail with reference to the accompanying drawings.